This invention is directed to electrode assemblies and, more particularly, to electrode assemblies for use with fuel cells having solid polymer electrolytes. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
A fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel and oxidant are stored external to the cell, which can generate power as long as the fuel and oxidant are supplied. The present invention relates to fuel cells in which the fuel is hydrogen and the oxidant is air or oxygen. Protons are formed by dissociation of H.sub.2 at the anode and pass through the electrolyte from anode to cathode. Electrons produced in the dissociation flow in the external circuit to the cathode, driven by the difference in electric potential between the anode and the cathode and can therefore do useful work.
Fuel cells can be classified by the type of electrolyte, i.e., liquid or solid, that they contain. The present invention relates to fuel cells in which the electrolyte is a solid polymer, also known as a proton exchange membrane. One copolymeric ion exchange membrane used as a fuel cell solid electrolyte is a perfluorocarbon material sold under the trademark Nafion.RTM. by E. I. duPont Nemours. The use of Nafion.RTM. as a solid polymer electrolyte membrane is more particularly described in U.S. Pat. No. 4,469,579, incorporated herein by reference.
Fuel cells using a solid electrolyte offer several potential advantages over liquid electrolyte fuel cells. These advantages include greater ultimate power density, lower operating temperature, and longer lifetime. Solid electrolytes also have the advantages of lack of corrosion, ease of construction, and low vapor pressure. Nafion.RTM. particularly provides improved oxygen reduction kinetics.
The anode and cathode half-cell H.sub.2 and O.sub.2 reactions require catalysts to proceed at useful rates. In acid electrolyte fuel cells a suitable catalyst is platinum, or one of the other noble metals such as palladium. The catalyst is provided as very small particles (20-50 .ANG.) which are distributed on, and supported by, larger microscopic carbon particles to provide a desired catalytic loading. Electrodes are formed from the catalyzed carbon particles and designed to optimize contact between the electrolyte, the gaseous fuel and oxidant materials, the catalyst, and an electrically conducting current collector. The electrodes must be porous where gaseous materials are involved and efficient porous gas diffusion electrodes have been developed for fuel cells using a liquid electrolyte such as phosphoric acid. These electrodes require relatively small quantities of the expensive catalyst materials. Platinum loadings for use with liquid electrolytes are about 0.3-0.5 mg/cm.sup.2 of electrode area.
Supported catalyst porous gas diffusion electrodes have not been successfully used with solid polymer electrolyte fuel cells, since only very low current densities have been obtained. The electrodes presently used with solid polymer electrolyte fuel cells are constructed differently than liquid electrolyte electrodes. Instead of using very small particles of noble metals, dispersed on a high surface area carbon support, a catalyst/carbon mixture is pressed directly into the surface of the polymer electrolyte. The catalyst loadings with these conventional electrodes for use with solid polymer electrolytes are about ten times higher than the catalyst loadings for porous gas diffusion electrodes used with liquid electrolytes. The great disparity in cost between these two types of electrodes has prevented the realization of the advantages offered by solid polymer electrolyte fuel cells for gas reaction fuel cells.
U.S. Pat. No. 4,469,579 teaches an electrode for use with a sodium chloride brine cell formed from a solvated perfluorocarbon copolymer blended with conductive electrode materials, and may include an ingredient to form pores for passing gases arising from the brine reaction products. A resulting solute dispersion, including relatively large (3.times.3.times.3 mm) carbon particles, can be formed as a sheet for application or can be applied directly to a solid polymer separator membrane. While the field of use for the electrode assembly suggests application to fuel cells with liquid reactants, there is no example of such application or of any benefits which might be realized. There is also no teaching about forming a porous structure suitable for use with gaseous fuel materials.
In conventional gas reaction fuel cells, a liquid electrolyte can intimately contact the catalyst and the gas for efficient ion generation and conductive transfer. Relatively low catalytic loadings are needed, e.g., loadings of 0.35 mg/cm.sup.2 are typical. Where a solid polymer electrolyte is used, electrode materials, such as a carbon and platinum mixture, have been pressed directly into the surface of the polymer which contacts a liquid reactant. Relatively high catalytic loadings are required, e.g., loadings of 4 mg/cm.sup.2 are typical.
The present invention overcomes problems associated with the application of porous gas electrodes to solid electrolyte electrode assemblies and an improved porous electrode structure and a method for forming the structure are provided.
One object of the present invention is to increase the proton transport between the catalytic surfaces within a porous gas diffusion electrode and a solid polymer electrolyte.
One other object of the present invention is to provide a fuel cell electrode having a greatly reduced amount of noble metal catalyst needed for electrodes used with solid polymer electrolyte fuel cells.
Another object of the invention is to successfully use porous gas diffusion electrodes with a solid polymer electrolyte.
Still another object is to enable the conversion of conventional porous gas diffusion electrodes for use with a solid polymer electrolyte.